1. Field of the Invention
The present invention relates a coding/decoding method for recording data in a high-density storage device, and an apparatus therefor, and more particularly, to a coding/decoding method for recording data with high density in a partial response maximum likelihood (PRML) channel, and an apparatus therefor.
2. Description of the Related Art
Currently, due to the increase in the amount of data to a be processed, much higher recording densities are required in storage devices. To achieve this objective, the data to be recorded in the storage device is encoded to increase the recording density and facilitate the detection of a reproduction signal. Also, as the recording density in the storage device increases, large quantities of useful information can be recorded on a storage disk having a predetermined size. Thus, an effective coding method is required such that the recording density is increased and detection of signals is facilitated, by applying less redundancy.
In general, as a useful coding method for a storage device, there is a run length limited (RLL) method satisfying a (d,k) condition. The aim of this coding method is to reduce interference between signals in order to detect a data signal while maintaining a self-clocking characteristic of a data signal. That is, according to the RLL(d,k) coding method, the number of successive "0"s between "1"s is limited to the minimum d and the maximum k. The former d is for facilitating signal detection, and the latter k is for maintaining the timing for restoring a reproduction signal.
As recently used coding methods using the RLL code there is the rate 1/2(2,7) modulation code, rate 2/3(1,7) modulation code, rate 8/9(0,3) modulation code, rate 8/9(0,4/4) modulation code and others. According to the rate 1/2(2,7) modulation code and rate 2/3(1,7) modulation code, "d" is equal to 1 and 2, respectively. Accordingly, interference between signals is decreased while redundancy is high due to the low code rate. Thus, more number of bits must be recorded in order to transmit the same amount of user data, thereby further causing inter-symbol interference (ISI) compared to a code having a high code rate, such as 8/9 RLL (0,3) code or 8/9 RLL(0,4/4) code.
In designing modulation code, it is important to provide a high code rate. The merits of the modulation code having a higher modulation code are that it increases a channel input signal-to-noise ratio (SNR) and there is less redundancy, thereby reducing interference between data for high-density recording.
Generally, when recording or reproducing data in or from a storage device, a channel must be modeled similarly to an actual channel. Channel characteristics of the storage device can be expressed by the following equation (1). EQU (1+D).sup.n or (1-D)(1+D).sup.n (1)
where n=1, . . . , 2.
Partial response maximum likelihood (PRML) pre-codes an input signal to provide controlled ISI between a current data and a previous data, and then modifies the same into the following target response (2), and data is detected using a Viterbi decoder. EQU d.sub.k =a.sub.k +a.sub.k-1 or d.sub.k =a.sub.k -a.sub.k-2 (2)
The PRML method exhibits excellent detecting capacity in a channel in which n=1. As data is recorded in a high density on a storage device, the distance between state transitions is made short, so that the ISI between data becomes serious. In order to reduce the ISI at high density recording, a method for reducing the state transition is provided. In a rate RLL(1,7) code adopting such concept, the coding is performed such that at least one zero exists between symbols. However, even though the rate RLL(1,7) code restricts the state transition, the code rate is low, so that more bits must be recorded compared to the rate RLL(0,3) code or the rate RLL(0,4/4) code having a code rate of 8/9 in order to transmit the same amount of user data, thereby reducing the interval between the recorded data. As a result, interference between data is increased and the merits obtained by restricting the state transition are lost.
When designing modulation codes, the most interesting factor is to provide a high code rate when a user data (binary data symbol) is mapped into a codeword. Such a high code rate is for increasing a channel input signal-to-noise ratio (SNR). Also, a code having a higher code rate reduces the interference between data in contrast to a code having a lower code rate, thereby reducing non-linearity to achieve high density recording.
As a coding method for reducing the ISI, there is a method for increasing the code rate and a method for spacing between the state transitions. However, these two methods are in a trade-off relationship. That is, the interval between the state transitions cannot be at a predetermined level or more without sacrificing the code rate, and it is also difficult to increase the code rate while the interval between the transition states is maintained. The method for reducing the ISI by spacing the state transitions is adopted in the rate 2/3 RLL(1,7) modulation code.
On the other hand, a rate 8/9 RLL(0,3) code and a rate 8/9 RLL(0,4/4) code are designed based on different concepts from the above code. According to these coding methods, the channel input SNR is high due to its high code rate, in contrast to the coding method in which the code rate is low. Also, less data is recorded compared to the coding method in which the code rate is low, in order to record the same amount of user data. Thus, the ISI is reduced, thus lowering the non-linearity.
However, the problem of ISI becomes more serious in the current situation where the recording density in a data storage device gradually increases. To achieve high density recording in a data storage device, the maximum transition run (MTR) code has been introduced. According to the coding method, the coding method for spacing the state transitions by one sample or more at minimum is slightly allowed. That is, the MTR is limited to 2. Also, instead of allowing the MTR, the code rate which influences the channel input SNR is increased. Thus, the MTR coding method is suitable for high density recording on a data storage device.
However, as storage devices require higher recording density, a coding or decoding method for a high-density recording or reproduction is required, in which the ISI is less than the conventional MTR coding method and the code rate affecting the channel input SNR is increased.